Older central processing units (CPUs) did not typically integrate platform components, such as memory controllers, into the same package as the processor itself. To communicate with such non-integrated components, the operating system (OS) might use a variety of schemes, including a register-based interface, such as the peripheral component interconnect (PCI) 2.0 interface. For more information on the PCI 2.0 interface, please refer to Revision 2.2 of the corresponding specification as published by the PCI-SIG (Special Interest Group) on Dec. 18, 1998, and later revisions.
To improve performance, some platform components, such as memory controllers, have been fabricated to reside in the same package as the processor core(s). To simplify such designs and increase performance still further, the registers of some integrated platform components have migrated from traditional 32 bit input/output (I/O) space into 64 bit memory-mapped address space. However, legacy (older) OS software used with such integrated platform components may still attempt to access the component registers using the older register-based mechanisms, such as the PCI interface.